A user device may register with a network (e.g., a cellular network) in order to place and/or receive telephone calls. The registration of the user device may be initiated by the network or by the user device. For example, the network may initiate registration of the user device when an incoming call is placed to the user device and when the user device is powered off or otherwise disconnected from the network. The network may register the user device in order to assign a call service control function (CSCF) server to the user device so that the CSCF server may process the incoming call (e.g., by forwarding the call to voicemail, application server, another user device, and/or to some other network). Alternatively, the user device may itself initiate the registration when the user device powers on a radio used to connect to the network and/or when a current registration for the user device expires.
When the user device registers with the network (e.g., after the network initiates the registration or after the user device itself initiates the registration), the particular CSCF server that may be assigned to the user device may be a CSCF server that was previously assigned to the user device when the user device was previously registered with the network.